the stability of ca-na pyroxene in low-grade metabasites
TRANSCRIPT
American Mineralogist, Volume 70, pages 16-29, 1985
The stability of Ca-Na pyroxene in low-grade metabasitesof high-pressure intermediate facies series
SHrceNonr Menuveprer AND J. G. Llou
Department of GeologyStanford University, Stanford, California 94305
Abstract
Minor metamorphic Ca-Na pyroxene occurs as minute crystals and as thin rims aroundprimary augites in metabasites intermediate between the prehnite-pumpellyite (PP) andpumpellyite-actinolite (PA) facies in the Sanbagawa belt, Shikoku, Japan. Their composi-tions vary systematically with mineral paragenesis, from augite to acmite, with a constantjadeite component of about 20 molVo. The Ca-Na pyroxenes in the pumpellyite-bearingassemblages contain a very low acmite component (AcaAugssJd2s), which increasesthrough Ac25Aug55Jd2s in the alkali amphibole-bearing and Ac66AugzsJdrs in the hematite'bearing assemblages, finally attaining an aegirine composition (Ac65Aug1sJd25) in themagnetite-bearing assemblages. They are distinguished from relict igneous augites chemi-cally by very low OPX and TiO2 contents, and morphologically by fibrous crystal habit.
The pyroxene-bearing metabasites contain assemblages of chlorite + pyroxene +pumpellyite (+ epidote, actinolite, alkali amphibole, stilpnomelane, hematite and magne-tite), which are incompatible with the higher grade and widely developed pumpellyite-actinolite facies assemblage, epidote + chlorite (Xr" : 0.50) + actinolite + pumpellyite.The reaction: augite + chlorite : pumpellyite * actinolite + H2O has been suggested forsuch a transition. The stability relations of augite + chlorite + pumpellyite were analyzedfrom comparative mineral parageneses in low-grade metabasites from New Caledonia,New Zealand, and the Appalachians. The assemblage may have a wedge-shaped P-T spacebounded against the high-Tpumpellyite-actinolite facies by the above reaction and againstthe low-T prehnite-pumpellyite facies by the reaction prehnite * actinolite = augite +chlorite with probable negative slope. From the mineral parageneses in the metabasites ofthe Sanbagawa belt and elsewhere, it is concluded that the Ca-Na pyroxenes occur within arestricted P-T space between the prehnite-pumpellyite and pumpellyite-actinolite facies.
Introduction
Clinopyroxenes are common in high-grade metamor-phic rocks such as amphibolite, granulite and eclogite andalso appear in blueschist facies metabasites. The charac-teristic clinopyroxenes in amphibolite and granulite areaugitic to diopsidic in composition whereas those ineclogite and blueschists contain jadeite or jadeite-richomphacite (e.g., Deer et al., 1978). Recently, sodic salitehas been described in meta-syenites of the albite--epidoteamphibolite facies (Droop, 1982). Diopsidic pyroxenesare common in some deep geothermal drill hole samples(e.9., Cerro Prieto by Bird et al., 19E4 and Schiffman etal., 1984, and Larderello by Cavarretta et al., 1982). Inspite of these occurrences, clinopyroxenes are typicallyabsent in rocks of low or medium metamorphic grade. Ifthey occur in metabasalts of zeolite, prehnite-pumpel-
rPresent address: Department of Earth Sciences, ToyamaUniversity, Toyama 930, Japan.
0003-004)085/0 I 02-00 I 6$02.00
lyite or even greenschist facies, they are generally regard-ed as relict igneous augites (e.g., Coleman, 1977).
This paper describes some unusual occurrences ofCa-Na clinopyroxene metabasites from the lower zone of thepumpellyite-actinolite facies from a small area of theSanbagawa belt, Japan. Relict primary igneous augitesalso occur in these metabasites. Evidence from textures,chemical characteristics, and phase equilibria describedin this paper indicates that the Ca-Na clinopyroxeneswere formed during the Sanbagawa metamorphism atconditions between the prehnite-pumpellyite and pum-pellyite-actinolite facie s.
Field relations
The Ca-Na pyroxene-bearing metabasites occur in asmall area in Kochi, central Shikoku, Japan (Fig. l).Regional geology, metamorphic zones, and mineral para-geneses and chemistries in adjacent areas have beendescribed by Banno (1964), Nakajima et al. (1977), andAiba (1982).
l 6
The Sanbagawa-Chichibu terrain is composed mainlyof pelitic-psammitic and mafic schists with subordinateamounts of quartzitic and calcareous rocks. Fossil agesrange from Late Carboniferous to Early Jurassic (Kan-mera, 1969; Matsuda, 1978, Isozaki et al., l98l); radio-metric ages for metamorphism range from 110 to 67 m.y.(Ueda et al.,1977).
Banno (1964) concluded that the Sanbagawa metamor-phism has affected the northern halfofthe Chichibu belt;the metamorphic grade decreases gradually from theepidote amphibolite facies in the north to the blueschistfacies in the south. In between, 4 metamorphic zonesfrom high to low grades have been delineated: biotitezone and garnet zone (both based on metapelite assem-blages); pumpellyite-absent zone and pumpellyite zone(both based on metabasite assemblages). A wide-spreadoccurrence ofthe prehnite-pumpellyite zone to the southof the boundary between the Sanbagawa and the Chi-chibu metamorphic terrain has been recognized by Hashi-moto et al. (1970). As shown in Figure l, E-W metamor-phic zones of the greenschist, pumpellyite-actinolite andprehnite-pumpellyite facies and the Kurosegawa tectoniczone are subparallel. The study area is located within thepumpellyite-actinolite zone about 2 km north of theboundary between the pumpellyite-actinolite and preh-nite-pumpellyite zones.
The Mikabu Carboniferous ophiolite complex occurswithin the study area between the Sanbagawa and theChichibu belts. It is 4 x 50 km in dimensions and consistspredominantly of mafic lavas and breccias (hyaloclas-tites) with subordinate amounts of ultramafic, gabbro,diabase and radiolarian chert (Fig. l). The ophiolitecomplex is bounded to the north by a fault and isconformable with the overlying clastic rocks of the Chi-chibu belt to the south. However, metamorphism tookplace after its emplacement, and the metamorphic gradeis continuous across the fault. The study area, therefore,contains a small section of Sanbagawa metamorphiczones. We focus our study on paragenesis and chemistryof minerals from the metabasites of the ophiolite com-plex, with emphasis on the paragenesis and stabilityrelations of clinopyroxene-bearing assemblages. Mineralparageneses of the prehnite-pumpellyite, augite-chloriteand pumpellyite-actinolite zones a.re shown in Figure 2.
Petrography of metabasites
Original structures (pillows, hyaloclastites, and feederdikes) and textures (ophitic, subophitic, intersertal andintergranular) are well preserved in the metamafics fromthe studied area. Primary plagioclase is completely al-tered to albite associated with pumpellyiter-phengite-r-epidote. Some chlorite pseudomorphs after olivine arerecognized by characteristic olivine crystal forms withtiny inclusions of spinel (mostly altered to magnetite).Relict diopside-augites are common and are replacedpartially by metamorphic Ca-Na pyroxene, actinolite, orsodic amphibole along cleavages and irregular cracks.
- S . r o r r r . r d s . r / \ . .
r-1:;-, "*-.t1y:viR y o k "
S a o b a g & { a
B € I r
Fig. l. Location of the study area and distribution ofgreenschist (GS), pumpellyite-actinolite (PA) and prehnite-pumpellyite (PP) facies rocks in central Shikoku, Japan(Hashimoto et al., 1970: Banno et al., 1978). The Mikabuophiolite complex is located within the study area.
Igneous oxides are completely recrystallized to spheneand have preserved their crystal forms.
Mineral assemblages from 250 thin sections were iden-tified. The assemblages can be divided into three groupsbased on the occurrence of calcite and metamorphicpyroxene: (1) calcite-free and pyroxene-free l2l0), (2)calcite-free and pyroxene-bearing [35], and (3) calcite-bearing and pyroxene-free [5]. The numbers in brackets
Schemolic Minerol Porogeneses ior metobosiles in Cenlrol Shikoku,Jopon
-=\ zoneMinerol -_-\ Prchnite -Pumpel ly i le Augite -Chlor i te Pumpel ly i te- Acl inol i le
prehnrle
pumpel ly i le
o c I n o l r i e
crnopyrorene
e p r d o t e
ol bi t6
c h l o r i t e
Reterences Hoshifroto el ol( r9701 this sludy lhis sludy B Nokolmo
e l . o l . I r 9 7 ? )
Fig. 2. Schematic mineral parageneses for metabasites of theprehnite-pumpellyite, augite-+hlorite and pumpellyite-actinolitezones in central Shikoku, Japan. Stilpnomelane, hematite,magnetite, sphene and muscovite may occur as accessoryphases.
0 l s k h:
MARUYAMA AND LIOU: Ca_Na PYROXENE IN LOW-GRADE METABASITES
l 8 MARUYAMA AND LIOU: Ca-Na
Table l. Mineral assemblages of metabasites*
PYROXENE IN LOW-GRADE METABASITES
lane-calcite and chlorite-calcite assemblages have signifi -
cance for our later discussion ofX6q" the fluid phase.
Mineral chemistry
Compositions of minerals were analyzed using a Hita-chi microprobe at Kanazawa University, Japan. Repre-sentative analyses are listed in Table 2. Complete analy-ses of minerals are available from the senior author.
Pyroxene
Metamorphic pyroxene occurs as an epitaxial fringearound and metasomatically replacing the igneous au-gites, and rarely as discrete fibrous crystals in the vein ormatrix. Some pyroxene occurrences are shown in Figure3. In most rocks, the relict augites are nearly colorless topale brown, whereas the metamorphic aegirine-augiteand aegirine ubiquitously show light green to pale greencolors. Some metamorphic augites are the same color asigneous augites but can be differentiated by distinctdifferences in chemical composition and crystal morphol-ogy. The igneous pyroxenes are characterizedby low Caand high Ti- and Cr-contents and the metamorphic pyrox-enes commonly have anhedral fibrous crystal forms. TheFe2O3 contents ofpyroxenes were calculated on the basisof a total of 4 cations for 6 oxygens. End-members werecomputed following the method of Banno (1959).
Figure 4 is a plot of analyses of both igneous andmetamorphic pyroxenes in terms of TiOz content andmajor pyroxene components En-Fs-Di-Hd. All meta-morphic pyroxenes contain less than 0.03 Ti atoms performula unit and less than 0.01 wt.% Cr2O3, whereasigneous pyroxenes either are chromian diopside withCr2O3 up to 1.07 wt.Vo or have greater than 0.02 Ti atomsper formula unit. Igneous pyroxenes have much morerestricted augite compositions in pyroxene quadrilateralplots than the metamorphic pyroxenes, which are charac-teristically higher in Fe and lower in Mg contents and arehighly variable in composition depending on mineralassemblages. The results of end-member calculationsindicate that more than 98%o of the components of themetamorphic pyroxenes can be expressed by the Aug (Di+ Hd)-Jd-Ac system in Figure 5. Three features areapparent in Figure 5: (l) Although considerable composi-tional scattering due to chemical zoning is shown inFigures 4 and 5, pyroxene compositions vary systemati-cally according to assemblages. Such variation suggeststhat chemical equilibrium may have been nearly attainedduring metamorphism. However, pyroxenes of the high-variance assemblage epidote-chlorite-pyroxene contain 0to 30 molVo acmite component. The pyroxenes coexistingwith epidote-chlorite-pumpellyite-actinolite are restrict-ed in composition to close to the augite-jadeite join. Thecrossite- and riebeckite-bearing metabasites contain py-roxenes intermediate in composition between augite andacmite. Pyroxenes coexisting with hematite or magnetiteare most acmite-rich. (2) The maximum jadeite content of
A. 9-4Il9!-qle: !!4 lllglelg-_ir@ slldLages
E p i d o t e - ( h l o r l t e - a c r i n o l i t e - p u n p e l l y i t e
C h l o r i t e - p u m p e l l y i t e r i e b e c k i t e + s t i l p n o n e l a n €
E p i d o t e - c h l o r i t e - p , r n p e l l y l t e + s E i l p D o m e l a n e
B C a
Epido Ee-ac t ino l i te+punpb l I y i te
E p i d o t e - r i e b e c k i t e + s E l l p n o n e l a n e
E p i d o E e - p u n p e l l y i t e + r i e b e c k i t e
Dp ido le - r iebeck i te+s t i lpnomelane+magnet i le
Ep ido te-a . t ino l i te -pumpel l y i Ee+hemat i le
r o x e n e - f r e e a s s e m b l
C h l d r i t e - r l e b e c k l r e + e p i d o t e
Puhpe l L y i te - r iebeck i re
E p i d o E e - c h l o r i t e - p u n p e l l y i l €
A l l a s s e h b l € g e s i n . l u d e a l b i E e , q u a r E z a n d s p h e n e ( + p h e n g l t e a n d a p a c i t e ) .
refer to the total number of thin sections investigatedfrom each group. Observed assemblages are listed inTable 1 and include minerals observed within a single thinsection, although Zen (1974) restricted stable assem-blages to those within a domain less than I mm wide. Thesize of equilibrated domains for Sanbagawa metamorphicrocks has been discussed by Nakajima et al. (1977).Theyconcluded that equilibrium is almost reached for mineralswithin a thin section, except for vein minerals of a laterstage or actinolite needles within albite prisms. For thepresent study we adopt the conclusion of Nakajima et al1977).
Minor phases include stilpnomelane, hematite, magne-tite, pyrite and chalcopyrite. Carbonate was determinedto be calcite rather than aragonite by testing with Feigl'ssolution. Sphene and phengitic mica are ubiquitous.Characteristic features of each group are described be-low:
Most metabasites (210 samples) from the study areacontain neither calcite or pyroxene.2 Mineral assem-blages of this group are further subdivided into twosubgroups; the first is characterized by epidote-chlorite-actinolite-pumpellyite, and the second by pumpellyite-riebeckite. Stilpnomelane appears only in the secondsubgroup. All analyzed chlorite have Fe/(Fe + Mg) ratiosmaller than 0.68; the chlorites in the first subgroup arelower in Fe than those in the second subgroup.
Most of the calcite-free and pyroxene-bearing metaba-sites contain fewer than five coexisting phases, althoughtwo sections contain 6 phases. The characteristic featureofthe assemblages is the stable association ofpyroxene *chlorite. Among calcite-bearing metabasites, stilpnome-
2Unless specified, pyroxenesmetamorphic origin.
described in this paper are of
MARUYAMA AND LIOU: Ca_Na PYROXENE IN LOW.GRADE METABASITES l9
most pyroxenes, regardless of mineral assemblage, isabout 20 molVo. (3) Chemical zoning of most clinopyrox-ene crystals is of a progressive type, i.e., the pyroxenerims are enriched in jadeite component compared to thecores. Some zoned pyroxene grains may contain lessjadeite rim; such features will be described later.
Epidote
Epidote is a common phase in these metabasites.Analyzed epidotes vary in pistacite component from lg to36 molVo, with a maximum frequency at 33-34Vo, asshown in Figure 6. Maximum pistacite contents (35-36%)occur in epidotes which coexist with hematite; epidotes inmagnetite-bearing rocks have pistacite contents of lg-27%. Such epidote compositions are consistent withthose from lower pumpellyite-actinolite facies rocks else-where in the Sanbagawa belt (e.g., Nakajima, 1982).
Actinolite
Actinolite occurs as fibrous aggregates around relictaugite and crossite-riebeckite, or as independent prismat-ic needles in the fine-grained metabasite matrix. Someactinolites are close to the ideal formula, Ca2(Mg,Fe)5Si6O22(OH)2. Actinolites coexisting with alkali-am-phibole, however, may contain up to 37Vo alkali-amphi-bole end-member. We estimate the Fe2O3 content of theamphiboles by assuming zero A-site occupancy. Thisassumption is justified for the metamorphic grade oflower pumpellyite-actinolite facies, by the extremely lowpargasite contents in amphiboles from quartz-bearingrocks, and by the calculated Si content of 7.48-8.02 forthe analyzed Ca-amphiboles. The compositions are plot-ted in a ternary diagram in terms of actinolite, glauco-phane and riebeckite end-menbers in Figure 7; the resultssupport the existence of the compositional gap that wasdocumented by Coleman and Papike (1968) and Toriumi(1975). Intermediate amphiboles between riebeckite andactinolite with X1a, = 0.71 occur in some metamaficsfrom the adjacent Omoiji-Kogawa area (Nakajima et al.,1977) and are shown by the closed circles in Figure 7.Relations such as those shown in Figure 7 suggest that themetamorphic grade of the clinopyroxene-bearing metaba-sites from the study area is slightly lower than that of theOmoiji-Kogawa area.
Crossite-riebeckite
This amphibole also occurs as fine-grained prismaticcrystals around relict augite. It shows distinct pleochro-ism from blue to violet and contains between 7 and 50 molVo of the glaucophane endmember. Some sodic amphi-bofes entirely rimmed by actinolite contain up to 35Voactinolite endmember (Fig. 7).
Pumpellyite
Pumpellyite occurs in amygdules, replacing plagioclasein veins with albite and/or quartz, and as fine-grained
aggregates in the matrix of metabasites. Compositions ofpumpellyites are plotted in Figure 8 in terms of 2Al-Fe*-Mg of Coombs et al. (1976) together with those of chloriteand stilpnomelane. Pumpellyite compositions vary interms of Al = Fe3* substitution; Mg contents also are notconstant, suggesting minor Mg = Fe2+ substitution. Thepumpellyite coexisting with stilpnomelane as shown inFigure 8 contains higher Fe3+ than other analyzed pum-pelleyites from the study area.
Chlorite
Chlorite is ubiquitous in these metabasites, and ishomogeneous in composition within a single specimen.Chlorites in ferrogabbro and plagiogranite tend to be deepgreen, whereas those in metabasalts are pale green. Suchdifferences may be due to diferences in Fe2O3 contentand in Fe/(Fe + Mg) ratio. The Sanbagawa chloritescontain about l0 wt.Vo total Fe as Fe2O3 (Banno, 1964).The analyzed chlorites range from pychnochlorite toripidolite through brunsvigite on Hey's (1954) diagram.They are plotted together with pumpellyite and stilpno-melane in terms of 2Al-Fe*-Mg in Figure 8. All chloritescontain nearly constant Al but show significant variationin Fe/(Fe + Mg) ratio. For pyroxene-bearing assem-blages, depending on the other coexisting phases, thisratio in chlorite varies systematically with pyroxenecomposition from 0.39 to 0.68 as shown in Figure 5.
Stilpnomelane
Stilpnomelane appears only in high FeO*/MgO metaba-sites, as brown needles in the matrix. Several analysesare plotted in Figure 8; they contain very high FeO astotal Fe and have rather restricted compositions in termsof 2Al-Fe*-Mg. It may be ferristilpnomelane in composi-tion, but may have been ferrostilpnomelane originally(Brown, l97l). The coexisting chlorite is dark green andhas an Fe/(Fe + Mg) ratio of about 0.68 and the coexistingpumpellyite is enriched in Fe content.
Paragenesis
The petrologic system of metabasites is discussed interms of the components SiOrTiO2-Al2O3-Fe2O3-FeO-MgO-CaO-Na2O-K2O-H2MO2. Quartz, albite, spheneand phengitic mica are ubiquitous in metabasites; they aretreated as excess phases for SiO2, Na2O, TiO2 and K2O,respectively. This simplification reduces the total numberof components to five: Al2O3, Fe2O3, FeO, MgO andCaO, assuming the fluid phase is in excess and consistsmainly of H2O in the calcite-free assemblages. If we fixthe chlorite composition at constant Fe/(Fe + Mg), thesystem can be further simplified to the four componentsCaO-Al2O3-Fe2O3-(Fe/Mg)O. The minerals treated hereare epidote, chlorite, pumpellyite, actinolite, stilpnome-lane, hematite, crossite-riebeckite and Ca-Na pyroxene.The first 6 phases plot within the tetrahedron in Figure 9,but the latter two plot outside. Acmite is plotted at an
20 MARUYAMA AND LIOT]: CA_NA PYROXENE IN LOW.GRADE METABASITES
Table 2. Representative chemical analyses of metamorphic and relict igneous materials
XimBl 6 l
l.eaI{.aP. b.
l c A ? u h ( r t )( fu02)
C P u S h(t505)
l C l P r k(t05t2)
21.90.00
1 9 . 8
29.10.49
1 3 . 20.000 . 1 2o . 0 !
t ! . t2
sio2 21.5TtO2 0.04i lzq 17.41.243P.o 24.tho 0.28rto 16. lcto 0.09l.2O 0.04I2O 0.05roirl E5.03
36.{ 52.1 54.0 35.50.19 0.05 0.0t 0.0?
2t, t t .35 t .99 22.21 5 . 2 2 6 . t
t 5 . 5 9 . 2 20 . 0 9 0 . r 8 0 . o 7 0 . t 3o . o { l 4 . r 7 . 5 0 r . 9 5
22.6 l l .5 0.90 22.2o . 0 l t . l 5 t . l A 0 . 0 ?o.02 0.09 0,02 0.02
96.45 95.02 98.43 9t.46
5 r . 80 . 0 5l . 1 8
15.2
0 . 3 9| , 1 3
1 7 . 64 . 2 4o. 04
,9.23
2 5 . 7 3 5 . 1 3 7 . 30.02 0,05 0.00
l 5 . l t 7 . 6 L 2 . 2
4 0 , { t 6 . , 3 5 . 50 . 2 r 0 . o l 0 . 2 t? , 0 5 l . l t 1 . 9 r0,06 2t. t 0.{30.00 0.00 0.050.o8 0.0t 0.25
06.63 91.48 93,91
5 0 . 30 . 1 3t . 0 t
34.7
o . 3 94 . 2 15.26t . 0 20.ot
100, t5
3 5 . t t 5 . 0 1 6 . l t l . 5o.ot 0.00 0.02 0.0t
2 1 . 6 0 . 6 ' 2 0 . 9 3 . t El ? . 0
! 2 . 9 l l . l 9 . 0 50.ol 0.14 0.06 t .420 , 0 t l 5 . t 2 , 6 5 1 0 . 3
22.9 t2.0 22.1 22.20.o4 0.55 0.o9 1.350.00 0.02 0.02 0.03
9t.4t 9?.50 '3.55 99.12
&tt.n
striAItr3+lc2+
hrtCaLI
2l
5.8870,0054,393
2.971 1.r l t - ' ,1t5 6.00? 2.0I7o.otz 0,0t 5 0,001 0.009 0.00t2.087 0.236 0.335 4.419 0.0540.932 2.885 0.521
4,144 1.92t 1.3020.051 0.006 0.022 0.00t 0.0t9 0.0t35.t93 0.005 t .r to r ,5qt t .49t 0.4480.020 1.994 1.t3, 0.13, 4.0t3 0.7340.0r, 0.oor 0.532 t .979 0.024 0.31t0.012 0.001 0.016 0.003 0.004 0.002
21.t
5.862 6,160 5.9rt t .Cr2o.qx o.o05 0.000 0.0044.065 3.54t 2.292 0.047
o.972|,?Ut 2.391 4.1280.041 0.002 0.029 0.0122.400 0.291 1.s12 0.459o . o l 5 a . o r 2 0 . 0 t 1 o . 2 t 00.000 0.001 0,020 o.otao.o22 o.rDl 0.052 0.m5
5,51t 2.915 1.92J 6.O2t 1.9t70.000 0.00t 0.001 0,002{ . 9 t 4 2 . 0 6 2 o . t l l 4 . u l 0 . 1 4 2
1.026t. t to t .54t l . t4 l 0.2410.ot9 0.002 0.01t 0.000 0.0454.197 0.001 t .45t 0. t12 0.51t0.01t 1.969 l . l5 l 4.000 0.9020.0t0 0,005 0.t60 0.02r o. loo.q!t 0.000 0,m3 0.m4 0.002
24.5 1 2 . t
L t
tuail.!.6p. fo.
l c l l l k(ar5u)
I C A I('1205'
I C A A r(a0r05)
l C t u h(t2!0r)
2 1 . 6 1 6 . 5 5 r , 60.03 0.05 0.5t
t8.{ 24.t 2.19
8io2riO2^r2%Fc203t OhotgoC.OhzolzoTobl
21.t 52.90.06 0.00
I r . 4 l . t 5
2 6 . 3 1 7 . 50.46 0.34
t4.5 t2.90 . 1 3 t o . t0 . 0 0 2 . 2 50 . u 0 . 0 9
86.3rt t7.2t
52.5 35.' 52.10.02 0.t8 0.04L U 2 0 . 2 2 . 0 t
24.5 t?.5 25.2
o . 2 5 0 . 1 3 0 . 2 23 . 5 4 0 . 6 3 2 . 1 6E.r2 22.5 5.49E . 6 l 0 . 0 3 1 0 . 20.0t 0,00 0.00
99.49 95.8t t00.73
21.1 36.2 35.60.m 0.34 0.07
r8. l 20.4 22.61 5 , I
5 3 . 0 3 t . 30.06 0.540 . 9 1 2 l . E
1 5 . rt2.r0 . 2 0 0 . 1 0
15.5 0.06l l .9 22.21.28 0.000 . 1 2 0 . 0 2
95.51 95.92
22.9 1 . 4o . 5 2 0 . 1 5 0 . 1 9
1 6 , 7 0 . 2 4 2 . O 21.06 20.9 22.50.00 0,00 0.000.02 0.05 0.03
85.52 94.39 90.85
t l . 80.34
1 9 . 50 . 3 70.030 . l l
85. l t
6 . 5 1 . 1 1o . l 2 o , l t3 . O 0 t 6 . l
22.6 2t.00 . 6 2 0 . 1 00.01 0.ol
92.6 9t.67
28. I0.02
1 5 . 9
22.10 . 2 6
t 7 . 60.0t0.0t0 . 1 2
*.62
fty3d
8iI tAItat+,c2+h
la&XrI
5.7t0 3.015 6.0380.001 0.02t 0.0094.520 2.000 6.496
1.00t4.049 1.0?60.091 o.ou 0.02t5.2eO 0.030 0.5080.246 1.870 4.0630.000 0.0@ 0.(x)00.005 0.006 0.@6
24.5
5.946 t .9t60.qr6 0.0144.76t 0, l t t9
0.474 0.2!9o.ol t 0.0050.721 0.E923.130 0.t3?0.196 0.00to.mt 0.001
6.09! 7.t30 3.Otlo.mt 0.006 0,021a.029 o.t t l 2.o7t
0.920l.9t t r . t7l0.047 0.025 0.m7t.690 3.420 0.0080.012 l . l to 1.9220.02t 0.35t 0.0000.012 0.022 0.003
2t.0
5.7930.0rr4t.554
3.1050.0606 . 1 0 50,o84o.olo0.029
24.5 t 2 . 5
5.t90 7.854 1,524 2.935 r.98tt0.006 0.000 0.002 0.011 0.0010.121 0.200 0.24t 1.950 0.0a9
2.519 t .0t9 0.74t4.143 2.16t0,0EE 0.042 0.031 0.009 0.00t4.69' 2.49 0.t8e 0.ot5 0.t550,0t0 1.604 t .326 1.5t6 0.2610,000 0.54, 2.425 0.005 0.7t90.029 0.0t7 0.001 0.0m 0.000
infinite point, and solid solution between augite andacmite is shown.
Calcite-free and pyroxe ne-free as semblages
These assemblages contain epidote, chlorite, pumpel-lyite, actinolite, hematite, crossite-riebeckite, and stilp-nomelane and are characteristic for the pumpellyite-actinolite facies. Depending on the bulk rock composi-tion, the assemblages of the two subgroups are plotted intwo tetrahedra. For the metabasites with chlorites of Fe/(Fe + Mg) ranging from 0.35-0.68, the mineral paragene-sis is shown in Figure l0A, and for the rocks having
chlorites with this ratio above 0.68 in Figure l0B. Thelatter is characterized by pumpellyite t stilpnomelane,and pumpellyite + riebeckite, which is incompatible withthe epidote-chlorite-actinolite plane in Figure 10A.Therefore, at Fe/(Fe + Mg) = 0.68, five phases epidote +
chlorite * actinolite + riebeckite + pumpellyite coexist.This 5-phase assemblage is divariant for the five-compo-nent system and hence the Fe/(Fe + Mg) of chlorite is afunction oftemperature and pressure. This ratio increaseswith increasing temperature as stilpnomelane occurs onlyin the lower grade part of the Sanbagawa belt, andpumpellyite does not coexist with alkali amphibole inhigher grade rocks (Banno, l9&).
MARUYAMA AND LIOU: Ca_Na PYR)XENE IN L)W-GRADE METABASITES
Table 2. (cont.)
2 l
kEp f , i Sr l (cce)
8io2 24.0 36.5TiO2 O.O2 0.O2l lzol r8,4 23.3Fclot l3.Zr.O Y.6h o o . 2 9 0 . 1 7rto 8.06 0.02&o 0.00 23.oxe2o 0.0O 0.04r2o 0.0t 0.02rorr l l6. l t 96,21
5 0 . t u . f 5 3 . 20.30 0.0t 0.o84 . 6 4 6 . 1 5 5 . 2 5
29.5 24.130.5
0 . 1 5 0 . 5 7 0 , 0 7h . 9 7 h . 3 9 0 . ? 02 . 2 0 0 . 2 3 2 . O 9, . 0 1 0 . r 3 1 3 . 7o . t t 0 . 6 3 0 . o 2
99.4E 87.71 t00.t l
2 9 . 00 . 3 4 0 . 1 7
1 2 , 8 0 . @0.00 22.10 . o 0 0 . 0 r0 , 0 r 0 . 0 2
t9,65 94.87
30. r0 . 1 6 0 . 8 26 . 1 4 5 . r 94 . 1 3 0 . O 06 . 3 5 0 . 0 40 , 0 3 t . 5 7
9 7 . 3 8 8 7 . 8 5
25.2 40.90.00 0.00
t 7 . 5 t 8 . 71 3 . 8
26.6o . 2 2 o . o t
t4.5 0.o50,09 22.80.07 0.15o . o t 0 . 0 r
85.29 96.55
52.1 50.30 . 5 1 o . 0 02 . 3 6 t . 3 9
t3.2 15.30 . r 9 0 . 3 3
1 5 . 3 9 . 2 7l l . t 2 t . r0 . t 3 3 . 7 50 . 1 I 0 . 0 0
95,2t 101.06
0 ,u
0.0t0 . 6 91 . 9 7
t3.20.02
98.8t
n.7 $.4 5o.8 43.t0 . 0 0 0 . o 7 0 . t 0 0 . 0 2
1 5 . 8 ? 0 . 0 4 . o 9 6 , 0 1l 5 . t 2 5 . O
5 0 . 90 . 1 22 . 5 2
t5.2o . t 89 . 5 0
t 5 . 05 . 9 00 . 0 2
99.34
r8.0ftJ'8.n
s iTiAIrc3+Pc2'hIgq
&
5.455 2.953 7.295 1.147 1.970 r.9670.003 0,001 0.032 0.@2 0.002 0,m34,93E 2.230 0.152 1.190 0.2t3 o.3or
0.805 3.203 0.5E1 0.6706.5950.055 0.012t.008 0.0020.000 2.00t0.0m 0.0060.002 0,002
4.2080 . 0 r 8 0 , 0 n 31.056 1.0r50 . 3 r 9 0 . 0 4 t1 . 9 5 t 0 . 0 4 20.020 0.173
6 , 3 E 3 3 . 0 1 8 7 . 4 0 10 . 0 0 0 0 . 0 0 5 0 . 0 1 14 , 1 1 0 2 . 0 3 0 0 . 7 0 3
0 . 9 4 3 2 . 1 4 65 . & 90 . 0 6 0 0 . 0 t 2 0 . 0 1 73 . 9 5 7 0 . 0 0 0 1 . 3 3 40.000 r.985 0.7390.000 0.00t 1.1960,003 0.002 0,005
1 . 9 6 7 5 . f 4 0 3 . 2 9 00.003 0.000 0.0000 . 1 1 4 4 . 5 3 t 1 . 7 7 1
0.8380 , 4 9 1 4 . A 1 30.006 0.042 0.0020.547 h,716 0.0050.622 0.020 t .9t0o . & 2 0 . 0 3 1 0 , 0 5 5o.oot 0.003 0.003
22
7 . 1 1 30 . 0 0 30 . 1 6 9
4 . 1 5 90 , l t 41.4490.{r00o . 0 1 40,330
2 1
l . 6 r l0.0670.404
0 . 6 1 90.0243 . 3 5 11.8090 . 2 0 60.020
5
1 . 9 4 3o.0000.063
0 . 4 9 3o . o l t0 . 4 7 60 . 9 0 00 . 2 t 10.000
o.0020.0t90 . 0 8 30.9820.001
o.0030.0380.0800.9640.00t
tel ict tu8tr .
AreJlq.8p. Xo.
l A C P u S(1504 )
c A P u k(t507) (t50r) (t602)
4 1 . 30.008.02
22
7 . 3 8 60,000t . 4 7 5
EiOzTi02AI203Fc203Cr203l€ohlgoCrOxrzolzolotr I
2 9 . t 5 1 . 00.05 0.00
1 2 . 8 2 . A 7
35.3 24.90 . 1 3 0 . 1 28 . E 5 5 . 5 7o . 5 5 1 0 . 50 . 1 3 l . , t0.o3 0. t0
87.04 98.67
6 . 5 3 6 7 . 7 5 50.008 0.0003.60t o.5t4
5.629 3,158o.o25 0.0t52.965 1.2620 . 1 5 t l , t l l0 . 0 5 8 r . 1 2 30.010 0.0t9
5t.5 t5.20 . 0 5 0 , 1 02,92 20.4
3 t . 5
12.40.05 0,074 . 0 7 t , 8 02 . O 2 2 0 . 86 . 2 2 0 . 0 30 . 0 2 0 . 0 2
98.35 90.82
3 1 . 7 t 4 . 60 . 0 0 0 . 0 9
l r . 5 t 3 , 7
3 5 . 7 t 9 . 80 . t 4 0 . 0 39 . 9 8 l . 1 90.53 2t.20 , 0 9 0 , 0 30 , 0 5 0 . 0 0
89,69 90.64
4 9 . 4 5 1 . 7o.(x 0.051 . 1 2 2 . 4 1
3 t . 9
28.9o . 2 3 0 . 0 75 . 2 h 5 . 5 19 . 2 1 3 . l 81 . 9 6 5 . 6 00 . o 8 0 . 0 1
96,t2 100.49
5 1 . 4 4 8 . 90 . t 8 2 . 1 62 . 1 5 7 . 5 3
1 . 0 7 0 . o 06 . 3 9 t r , 6o . l t 0 . 3 6
r7,0 t2.320.6 17.2o . t l 0 . 5 10.00 0.o0
99,47 100.55
1 . 9 0 7 1 . 8 1 60 , 0 1 1 0 . 0 5 00.094 0.330
0.031 0.m0o. l9E 0.3590 . 0 0 5 0 . 0 t t0.941 0.6800,628 0.5850.008 0.0370.000 0.000
28.60 . 2 9
0 , 3 tr . 5 81 . 6 5
9 1 . t 9
41.50 . t 02 . 3 2
0 . r Jt . 7 4
t 0 , 7l.O00 , t 0
9 8 , 7 t
7.495 5.0560.005 0,0140.500 4,1503.L45
1 , 7 9 t0 . 0 0 6 0 . 0 u0.842 0.4620.3t5 3.846t.?54 0.0090 . 0 0 4 . 0 . 0 0 3
6.815 6.298 7.78e0.o0 0.012 0.0042.9JO 2.939 0.319
7 . 3 8 5 1 . 9 8 20,006 0.0030 . 4 0 6 0 . t 1 63.4t2
3 . 7 3 t0.0t8o . 1 7 70 . 0 5 20.5090 . 3 2 8
t.2260.0050 . 1 0 8O.tr |10,08r0.1105
6.468 3.006 3.8070.026 0.009 0.03t 0.0093 . 2 2 4 0 . 3 2 4 r . 2 3 0 t . t 8 t0 . r 2 2 4 . r 3 3 1 . 5 6 5 0 . { 8 70 . 0 3 9 0 . 0 1 1 0 . 5 9 4 0 . t 5 10.013 0.000 0.015 0.002
. l I I t coQl .e ly .u r rdndd b t .c ! tm l i ! . .
C alc it e -fr e e a nd py roxe ne -b e arin g a s s e mbla g e s
This group includes the assemblage pyroxene * chlo-rite, which is incompatible with the topology in Figuresl0A and B of the above group. As shown in Figure 4,compositions ofpyroxene and chlorite vary according tomineral assemblage. With increasing acmite molVo inpyroxene, the coexisting chlorites change compositionfrom Fe/(Fe + Mg) = 0.35 to 0.68. For chlorite withFe/(Fe + Mg) greater than 0.68, the assemblage pyroxene+ chlorite is not stable.
Paragenetic relations for rocks with chlorite of Xpg
ranging from 0.35 to 0.68 are shown in Figure l0C, andare projected from chlorite onto a plane perpendicular tothe 2Al-Ca-FM plane and 80" inclined to the Ca-2Al joinaccording to Figure 9. Mineral assemblages of the pum-pellyite-actinolite facies are shown. Depending on thebulk rock Fe3+/Al ratio, assemblages of pumpellyite-actinolite, epidote-pumpellyite-actinolite, epidote-actin-olite, epidote-actinolite-riebeckite, hematite-epidote-riebeckite and hematite-riebeckite may occur. Superim-posed on these mineral compatibilities are the composi-tions of pyroxene solid solution between augite-acmite atconstant jadeite end-member of 20 molVo. Because the
22 MARUYAMA AND LIOU: CA-NA PYROXENE IN LOW-GRADE METABASITES
Fig. 3. Photomicrographs show the occurrence of some metamorphic pyroxenes in metabasalts from the study area. Scale bar is
0.1 mm. Abbreviations are Hb: hornblende; Ab: albite; M-CPX: metamorphic clinopyroxene; Aug: relict augite; MT: metamorphic
magnetite. A. Acicular needles of metamorphic Ca-Na pyroxenes growing into albite. B. Thin rims of Ca-Na pyroxenes growing
epitaxially along margin of relict augite. C. Metamorphic zoned Ca-Na pyroxenes growing into an albite * minor quartz vein. The
detailed compositional profile is shown in Fig. ll. D. Metamorphic pyroxene replacing the relict augites along margins or irregular
cracks. E. Discrete metamorphic pyroxene grain within albite together with phengitic mica and chlorite. F. Fan-shaped metamorphicpyroxenes intergrown with metamorphic magnetite.
MARUYAMA AND LIOI]: Ca_Na PYROXENE IN L1W-GRADE METABASITES
20 25 30 35
looXFe3*
23
A.
2
o o o B
M g
Fig . 4 . Cont ras t in chemica l compos i t ionsmetamorphic (closed circles) and igneous (openpyroxenes of the metabasites in the studv area.
be tweencircles)
augite-acmite solid solution is incompatible with thepumpellyite-actinolite assemblages, six reactions are ap_parent from the paragenetic relations and are writtenbelow:
Fe3*-poor Pum * Act + H2O: Chl + Acm-poor Augite
Ep + Pum * Act * H20 : Chl * Ca-Napx
Ep + Act + H20: Chl + Ca-Napx
Fig. 6. A. Compositions of epidotes in various assemblagesfrom the study area. B. Histograms showing compositions ofepidotes of a bufered assemblage epidote-chlorite (Fe/(Fe + Mg)= 0.a0-0.a5) - pumpellyite-actinolite-quartz in lower and upperpumpellyite-actinolite and greenschist facies metabasites (dataare from Aiba, 19E0, Nakajima et al., 1977 and Nakajima, 1982).
Ep + Acr + Ri + H20 = Chl + Ca-Napx (4)
Fe3*-rich Ep + Ri * Hem + H20= Chl + Acm-rich Px (5)
Ri + Hem + H20 = Chl + Acm-rich Px (6)
A c
o p r e s e n t r t u d y
. O E o i i i - K o g a w a
( N e k s j i E a e t a l . , l 9 ? ? )
G I
Fig. 7. Compositions of amphiboles in a ternary diagram ofactinol i te (Ac), glaucophane (Gl) and r iebeckite (Ri)components. Also shown is the composition gap by Toriumi(|975).
( l )
(2)
(3)
E C 0 3 9 - 0 4 5
E C A P 0 3 5 - 0 1 1E C S R 0 4 7 - 0 s s
E C R A H 0 5 0 - 0 5 3
E C R S M 0 6 7 - 0 5 8
N a - A lNa Fe
Fig. 5. Compositions of metamorphic pyroxenes plotted in aternary diagram of Ca (Di + Hd) - Na-Al (adeite) - Na_Fe(acmite) system. Abbreviations: E : epidote, C : chlorite, A :actinolite, p = pumpellyite, S : stilpnomelane, R : riebeckite,H : hematite and M : magnetite. Those numbers shown on theright top corner are the Fe/(Fe -| Mg) ranges of the coexistingchlorites.
R i
G a p b y
T o r i u E i ( 1 9 ? 5 )
24 MARUYAMA AND LIOU: CA-NA PYROXENE IN LOW.GRADE METABASITES
& FelFe+Mg in< o 6 8
c.
Fig. 8. Compositions of pumpellyite (pum) (large solid circle),chlorite (chl) (small solid circle) and stilpnomelane (st) (solidstar) plotted on a ternary diagram of 2 Al-Fe* (total Fe as FeO)-Mg. Coexisting compositions for pumpellyite-chlorite andpumpellyite--chlorite-stipnomelane are shown by tie lines.
From equations (1) to (6), pyroxene changes compositionfrom augite to acmitic augite. This relationship is consis-tent with the variation of clinopyroxene compositionshown in Figure 5. Because H2O is required for theformation of the pyroxene-bearing assemblages, pyrox-ene-chlorite assemblages are stable at lower tempera-tures than those of the pumpellyite-actinolite compatibili-ties.
Fig. 9. Composit ional relat ions of chlori te (chl),
stilpnomelane (st), pumpellyite (pum), epidote (ep), actinolite(act), glaucophane (gl), riebeckite (ri), augite (aug) and acmite(ac) in a tetrahedron of2 Al - 2Fe3+ - Ca-FM components. Aprojection plane perpendicular to the 2Al-Ca - FM plane andabout 80' inclined to the Ca-2Al join is shown.
Fig. r0. Minerat r*jJ;:r":T',rl".uou.n", rrom the studyarea and chlorite projection to show the pyroxene-formingreaction: A. Metabasite assemblages for the rocks with chloriteof Fe/(Fe + Mg) ratio ranging from 0.35-0.6E); B. For rocks withchlorite of Fe/(Fe + Mg) ratio greater than 0.68; C. Incompatiblepyroxene-bearing assemblages with those in A on a chlorite-projection diagram showing (a) the six reactions described in thetext, and (b) systematic variation of pyroxene composition alongthe join augite-acmite with variation in reactions.
In the northern, higher pumpellyite-actinolite faciesrocks, the pyroxene-bearing assemblage is extremelyrare. None was reported by Banno (1964) and only onepyroxene-stilpnomelane-bearing metabasite has been de-
scribed by Otsuki (1980). By contrast' to the southwest ofthe study area, pyroxene * chlorite assemblages havebeen described by Iwasaki (1963) and Aiba (1980). From
the previous and present studies, it is concluded that thepyroxene-bearing assemblage is stable at the lower gradepart of the pumpellyite-actinolite facies, with metamor-phic conditions roughly corresponding to those of the
naffow region of the present study area. The pyroxene-
bearing and pyroxene-free assemblages are randomlydistributed through the area, suggesting local diferencesin the activity of HzO; high agr6 favored the occurrenceof the pyroxene * chlorite assemblage'
C alc it e -b e arin g a nd py r o xe ne -fr e e a s s e mb Ia g e s
Rocks with high FeO*/MgO may contain a calcite-stilpnpmelane assemblage. For lower FeO*/MgO rocks,
calcite-chlorite-riebeckite assemblages occur, and theyare incompatible with actinolite-epidote or actinolite-pumpellyite as shown in Figure 10A. These relationssuggest that the appearance of calcite is controlled by
local increase of X6e, in the fluid. The value of Xse' in thefluid might be lower than 0.02 at about 300'C, judged fromthe common occurrence of sphene rather than rutile-calcite-quartz (Ernst, 1972). However, it should be em-phasized that only about 2Vo of the metabasites containcarbonate-bearing assemblages. Therefore' it is reason-able to conclude the fluid phase present during the
>o 68
MARUYAMA AND LIOU: Ca-Na PYROXENE IN L}W.GRADE METABASITES 25
pumpellyite-actinolite facies metamorphism of basalticrocks from the study area was highly aqueous.
Discussion
P-T path of Sanbagawa metamorphism deducedfrom zoning of clinopyroxene
Most examined Ca-Na pyroxenes exhibit progressivezoning with higherjadeite content towards the rim. A fewgrains contain a thin film of less jadeitic pyroxene at theoutermost margin. One example (Spl8ll) is shown inFigure ll. The pyroxene,0.8 x 1.0 mm in size, grew intoan albite-quartz vein. Fifty analyses for 9 elements werecompleted for this crystal, and the jadeite mol%o of theanalyses are shown in Figure I l. Jadeite componentincreases from 0.5-2.4Vo up to 5.3% jn the core region,and decreases gradually towards the rim. The outermostrim ranges from 0.4 to 2.3%. Such a distribution patternshows a progressive zoning for the core followed by aretrogressive one for the rim, and is not common inSanbagawa metabasites. It may be related to the vein-forming process whereby metamorphic recrystallizationadvanced rapidly due to sufrcient material migrationthrough veins. The same sample contains groundmassclinopyroxene grains with about 19 molVojadeite. Thesecompositional variations among clinopyroxene crystalswithin a single sample probably indicate local equilibri-um. Although local equilibrium may have prevailed dur-ing recrystallization, as discussed before, the systematicdifference in pyroxene composition suggests that themineral assemblages may have approached equilibrium.
The jadeitic pyroxene-albite-quartz sliding equilibriumas shown in Figure 12 is independent of the presence orabsence of a fluid phase. Thus we are able to estimate theP-Tpath of the Sanbagawa metamorphism without know-ing the exact values of agrs. The diopside-acmite-jadeite
Fig. I l Compositions of a zoned pyroxene crystal grown intoan albite-4uartz vein as expressed by molVo of jadeitecomponent (see Fig. 3C for the occurrence).
T"cFig. 12. The P-T path of the Sanbagawa metamorphism
deduced from zoned pyroxene coexisting with albite and quartz.
system, however, includes two miscibility regions be-tween omphacite (P2ln)_:a'ugite (C2lc) and omphacite-jadeite (Ah) @.g., Carpenter, 1980) and the ideal solu-tion model of Essene and Fyfe (1967) is not valid underthe conditions of low-grade metamorphism. However,immiscibility regions are closed towards the augite end-member under the lawsonite grade of jadeite-glauco-phane type metamorphic belts (Carpenter, 1980; Okay,1980). It is valid to discuss a P-T path, using pyroxenesliding equilibria only in the miscibility region. Figure 5shows the compositional range of Ca-Na pyroxenes inthe study area, and covers only the miscibility region.
The possible P-T path for the Sanbagawa metamor-phism is shown in Figure 12 together with the equilibriumline ofjadeite + quartz + albite line (Birch and Le Comte,1960) and calculated isopleths of 30Vo, 2Mo and, l}Vojadeite end-member by Essene and Fyfe (1967). Chemicalzoning shown in Figure I I indicates that initially bothpressure and temperature increased, then temperatureincreased with or without an accompanying pressuredrop, and finally both P and T decreased. The P-Itrajectory shown in Figure 12 is in good agreement withthose estimated from garnet<hlorite-biotite equilibria byHigashino (1975) and amphibole zoning by Otsuki (1980)for the Sanbagawa metamorphism.
Distribution of metamorphic facies in the San-bagawa belt in Shikoku
The metamorphic grade in central Shikoku generallydecreases from north to south. The highest grade metaba-sites contain albite-oligoclase-hornblende-epidote (En-ami, l9El) of the amphibolite facies. With decreasinggrade towards the south, the metabasites successivelyexhibit mineral assemblages of the albit+epidote amphib-olite, greenschist, pumpellyite-actinolite and finally preh-nite-pumpellyite facies (Hashimoto et al., 1970; Banno,
JadEite-Acmit€ Sol id solui ion
26 MARL|YAMA AND LIOU: CA_NA PYROXENE IN LOW-GRADE METABASITES
1977). Figure I shows the distribution of metamorphicfacies in the lower grade part of the Sanbagawa belt inShikoku through comparison of the metamorphic grade ofthe study area to those of surrounding areas.
Banno and his coworkers (Kurata and Banno, 1974;Higashino, 1975; Nakajima et al., 1977; Nakajima,1982)have defined several continuous reactions to relate suchchanges in mineral assemblages according to paragenesesand compositions of particular minerals of the bufferedassemblages. For example, Nakajima et aL. (1977) andNakajima (1982) delineated metamorphic grade using thecomposition of epidote in buffered assemblage, epidote-chlorite-pumpellyite-actinolite. The Xpe of chlorite isfixed at 0.45-0.50. Frequency distribution diagrams ofepidote composition show a systematic increase in Alcontent of epidote with increasing temperature (Naka-jima, 1982). Compositions of the analyzed epidotes fromthe buffered assemblage from our study area range fromPS 3l to PS 34 (Fie. 5), corresponding to the epidotecompositions of the lower pumpellyite-actinolite faciesand similar to those in metabasites from Omoiji-Kogawa(Nakajima, 1982) and from the Chichibu belt (Aiba, 1980,1982). The Fe*/Al ratio of pumpellyite in the sameassemblage has also been used to define the metamorphicgrade by Aiba, (1980, 1982); their results (Fe*/Al atomicratio : 24.0 - 32.6) are comparable to those in the abovetwo districts.
Figure 13 plots the partition coefficient fr$fff;-iffi1 fromthe upper pumpellyite-actinolite zone (Otsuki, 1980) andlower pumpellyite-actinolite zone (Nakajima et al., 1977 ;Aiba, 1980; Nakajima, 1982). The Kp's of the upperpumpellyite-actinolite zone range from 7 to 10, whereasthose in the lower pumpellyite-actinolite zone vary from3 to 6. The Kp's for pumpellyite/chlorite pairs from thestudy area range from 0.9 to 5 and indicate again thelower pumpellyite-actinolite facies. It is thus concludedthat the present study area, where metamorphic Ca-Na
pyroxenes occur, corresponds to lower pumpellyite-ac-tinolite facies.
P ump e lly it e-ac t inolit e fac ie s
Hashimoto (1966) proposed that the pumpellyite-actin-olite facies is an independent facies between the prehnite-pumpellyite metagreywacke and greenschist facies on thelower pressure side, and between the glaucophane schistand greenschist facies on the high-pressure side. Thissuggestion has been accepted by Seki (1969), Coombs(1971) and others.
Nitsch (1971) experimentally determined the stabilitylimit of epidote + chlorite + actinolite + pumpellyite +quartz + fluid, which occupies a wedge-shaped P-Tspace. The high-T and low-T sides are bounded, respec-tively, by the reactions: pumpellyite * chlorite : epidote+ actinolite * fluid, and prehnite + chlorite + fluid :
pumpellyite * actinolite. His experiments, however,were conducted using natural minerals as starting materi-al, and the determined P-T lines for the above reactionsthus are not univariant.
As described earlier, metabasite stability relations canbe simplified to a five-component system with excessquartz, albite and fluid (e.g., Zen, 1974; Banno, 1977).The high-T reaction of Nitsch is, therefore, trivariant. At
a given Fe/(Fe + Mg) in the system, the reaction has beentreated as divariant by Nakajima et al. (1977) and Naka-jima (1982). Hence, the epidote composition varies con-tinuously in P-T space. In the Sanbagawa belt, theboundary between the pumpellyite-actinolite and green-
schist facies is defined by the disappearance of pumpel-
lyite, which may coincide with epidote of PS : 15 for thebasaltic system (Nakajima, 1982). In other words, thesubstitution of Al by Fe3* remarkably affects the pumpel-
lyite-consuming reaction. They calculated the effect ofFe2O3 on the P-Tposition of the reaction, and their resultsindicate that equilibrium occurs 115'C lower for theFe2O3-saturated condition than for the Fe2O3-free sys-tem. The Fe2*/Mg substitution in actinolite and chloritemight also significantly affect this sliding equilibrium as
suggested by their partition coefrcients, although the
details have not been investigated.Nakajima et al. (1977) proposed subdivision of the
pumpellyite-actinolite facies by the discontinuous reac-tion: epidote(Ps : 33) + actinolite + chlorite (Fe/(Fe +
Me) : 0.45 - 0.50) + H2O : pumpellyite * hematite.The lower-T subfacies is characterized by the metabasiteassemblages pumpellyite + hematite * actinolite or pum-pellyite + hematite + epidote with excess chlorite, quartz
and fluid; epidote cannot coexist with actinolite. Aiba(1982) has, subsequently, confirmed the occurrence ofthese assemblages in the lower grade part of the San-bagawa belt, where Iwasaki (1963) and Aiba (1980) alsodescribed a Ca-Na pyroxene-chlorite assemblage insome metabasites.
In the previous sections, the Ca-Na pyroxene-chloriteassemblage is shown to be incompatible with a pumpel-
o . 2o . l
o . o 5
o . o I
o ) P a f a c i e s t o w e r
+ P r e s e n t S l u d y
o . o o , o . o t o . o s o . t
( M " / M c ) c h l o r i t e
Fig. 13. Mn-Mg partition coefficients between actinolite andchlorite from upper and lower pumpellyite-actinolite facies ofthe Sanbagawa metamorphic belt. Data for upper and lowerpumpellyite-actinolite facies are from Aiba (1980), Nakajima etal. (19'17) and Nakajima (1982).
MARUYAMA AND LIOU: Ca-Na PYROXENE IN L)W-GRADE METABASITES 27
lyite-actinolite assemblage in some metabasites. It occursin rocks which have been metamorphosed at lower pum-pellyite-actinolite facies or at conditions close to those ofthe discontinuous reaction proposed by Nakajima et al.(1977). This relationship suggests that the low-Zboundaryof the pumpellyite-actinolite facies could also be definedby a reaction different from those proposed by Hashimoto(1966), Nitsch (1971) and Seki 0971).
Stability field of Ca-Na pyroxene * chlorite *pumpellyite
The assemblage augite + chlorite appears to be stableat the P-T conditions defined near the above discontinu_ous reaction by Nakajima et aI. (19]7), and it can be usedto subdivide the pumpellyite-actinolite facies. Immedi_ately north of the study area occur the characteristicpumpellyite-actinolite facies assemblages. To the southofthe study area, augitic to acmitic pyroxenes have beenfound by Iwasaki (1963) and Aiba (1980). Further sourh,the prehnite-pumpellyite zone is developed (Hashimotoet al., 1970). Therefore, the augite * chlorite assemblageis stable at restricted conditions between those of thepumpellyite-actinolite and the prehnite-pumpellyitezones. Augite-chlorite zones have not been recognizedbetween the prehnite-pumpellyite and pumpellyite_actin_olite zones from either higher or lower plT facies series,such as New Zealand (Coombs, l97l; Kawachi, 1975;Coombs et al., 1976), the Appalachians (Zen, 1974), andNew Caledonia (Black, 1977; Brothers, 1974). Appear_ance of augite with chlorite may be restricted to metaba-sites of the high-pressure intermediate facies series.
At constant Fe3+/Al (pS : 33) in epidote and Fe/(Fe +Mg) (0.a5) in chlorite, the topologic relationships ofepidote, prehnite, lawsonite, pumpellyite, actinolite andaugite in low-grade metabasite in a ternary ACF systemare constructed. As chlorite is ubiquitous in metabasites,compositions of all these Ca-Al silicates are projectedfrom the Sanbagawa chlorite (Mg,Fe)sAlzSi4Or5(OH)r2onto the pseudo-binaryjoin A-C. There are 4 univariantlines radiating from each invariant point for the pseudo-binary system. However, for clarity, some invariantpoints are shown with only 3 univariant lines. Among l5invariant points (6Ca = l5), four are relevant ro ourdiscussion on the augite stability; they are shown inFigure 14. The first invariant point [Lw,Aug], from whichradiate four univariant lines, was estimated by Nitsch(1971) to occur at 2.5 Kbar and 350"C.
One of these lines is the reaction pr + Chl + F : Act +Pum, which has a negative slope and defines the bound-ary between the prehnite-pumpellyite and pumpellyite_actinolite facies. Mineral parageneses in New Zealandfrom the prehnite-pumpellyite zone grading directly tothe pumpellyite-actinolite zone without passing throughthe augite zone, can be defined by the above reaction. Athigher pressure, this reaction is terminated at anotherinvariant point [Ep,Lw], from which radiates anotherthree reactions; two of them extend towards higher
t [ t w , A u s ]
r t , [E p, L sl
l r l , t E p , p r J
r v , [ E p , p b ]C,-; : : : -- . : lF" Aug Ac t
.
Fig. 14. Possible P-Z stability field (shaded area) for theaugite--chlorite-pumpellyite assemblage in basaltic ACF system.The phase relations are constructed for a pseudobinary systemprojected from constant chlorite composition. Arrows indicatethe P-T trajectories for different metamorphic facies series. Forstability relations among the prehnite-pumpellyite, pumpellyite-actinolite, prehnite-actinolite and greenschist facies see Liou etal. (198fl .
pressures. These two reactions, as shown in Figure 14,define successive zones ofpumpellyite * prehnite, augite+ pumpellyite, and pumpellyite + actinolite with increas-ing grade. The boundary between the prehnite-pumpel-lyite and augite + pumpellyite zones is defined by areaction: prehnite + actinolite + H2O : augite + chloritewith a negative steep P-T slope. The boundary betweenthe augite-pumpellyite and pumpellyite-actinolite zonesis defined by a dehydration reaction: augite + chlorite :pumpellyite * actinolite + H2O with a positive slope.
In New Caledonia, a metamorphic zone characterizedby lawsonite-actinolite without augite develops on thelower-T side of the pumpellyite-actinolite zone (Brothers,1974). Therefore, an invariant point [Ep,Pr] might bestable between 6 kbar for the Sanbagawa metamorphism(Toriumi, 1975) and about 8 kbar in New Caledonia(Brown, 1977;Brothers and Yokoyama, 1982). Radiatingfrom the invariant point is a univariant solid-solid reac-tion: lawsonite + actinolite : augite * chlorite, whichdefines the high-pressure limit of the augite + pumpellyitezone. A similar stability field of augite + chlorite in thelower grade part of the Kanto Mountains, has beenrecently deduced from a complete solution of the ACFsystem with 7 phases: augite, chlorite, pumpellyite, glau-cophane, actinolite, lawsonite and epidote (Hirajima,1983).
It is apparent from Figure 14 that the small P-T space,in which the augite + chlorite + pumpellyite assemblageis stable, exhibits a wedge shape with decreasing pres-sure. This assemblage appears only in the low-grade part,between the prehnite-pumpellyite and pumpellyite-actin-olite zones, of the high-pressure intermediate facies se-ries.
28 MARTJYAMA AND LIOT]: CA_NA PYROXENE IN LOW-GRADE METABASITES
Conclusions
Several conclusions are deduced from the petrological-mineralogical study of some low-grade metabasites de-scribed in the previous sections. They are listed below:
L Minor amounts of Ca-Na clinopyroxenes occur asminute fibrous crystals, metasomatic replacements aswell as thin rims around primary augites in some metaba-sites between the prehnite-pumpellyite and pumpellyite-actinolite facies.
2. The Ca-Na clinopyroxenes are stable with chlorite+ pumpellyite t epidote l actinolite t sti lpnomelane-rcrossite-rhematitetmagnetite in these metabasites.
3. The Ca-Na clinopyroxenes are very heterogeneousin composition and contain much Iower CrzOyTiOzwt.Voand OPX mol7o than the relict primary augites.
4. The Ca-Na clinopyroxenes vary in compositionsystematically with their coexisting mineralogy; those inpumpellyite-bearing assemblages contain a very low ac-mite component (AcsAugs0Jdzo), and increase throughAc25Aug55Jd2e in the crossite-bearing and Ac6sAug25Jd15in the hematite-bearing, and to AcesAugroJdzs aegirine inthe magnetite-bearing assemblage.
5. Such a systematic change in pyroxene compositionis explained by a series of reactions deduced from (1)difference in paragenesis and compositions of minerals inthe pumpellyite-actinolite and prehnite-pumpellyite fa-cies metamafics, and (2) topologic relations of phases in abasaltic ACF system.
6. Composition of chlorite and epidote and Mn-Mgdistribution between pumpellyite and chlorite togetherwith field relations suggest that the clinopyroxene-bear-ing metabasites were recrystallized at P-T conditionsbetween the prehnite-pumpellyite and pumpellyite-actin-olite facies.
7. Stability of the augite-chlorite-pumpellyite assem-blage falls within a wedge-shaped P-T field that is bound-ed by a high-Treaction augite + chlorite : pumpellyite +actinolite + H2O and a low-I reaction prehnite * actino-lite + H2O : augite + chlorite.
Acknowledgments
The analytical work was completed at Kanazawa Universityunder the guidence of Professor S. Banno. Discussions with S.Banno, T. Hirajima, K. Aiba and T. Nakajima are most helpful.The manuscript was prepared at Stanford where the seniorauthor is a post doctoral research fellow supported by NSFEAR82-04298. We wish to thank Mary Keskinen, Peter Schitr-man and W. G. Ernst for critical review of this manuscript.
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